PNNL

Abstract

Potassium (K) is an essential nutrient yet has limited bioavailability in many environments. Microbes can biochemically weather minerals that are rich in K, however, the mechanisms underpinning microbial K processes, are poorly resolved. To better understand the controls on microbial K transformations we performed K K-edge X-ray absorption near-edge structure (XANES) spectroscopy on K-organic salts including acetate, citrate, nitrate, oxalate, and tartrate, which are frequently observed as low molecular weight organic acids secreted by soil microbes, as well as humic acid which acts as a proxy for higher molecular weight organic acids. The organic salts display feature-rich K XANES spectra, each demonstrating numerous unique features spanning ~ 13 eV range across the absorption edge. In contrast, the spectra for humic acid has one broad, wide feature across the same energy range. We used a combination of time-dependent density functional theory (TD-DFT) and the Bethe-Salpeter Equation (BSE) based approach within the OCEAN code to simulate the experimental spectra for potassium nitrate (KNO3) and K-citrate (K3(C6H5O7). H2O) to identify the electronic transitions that give rise to some of the outlying and unique spectral features in the organic salts. KNO3 has both the lowest and highest lying energy features, and K3(C6H5O7). H2O is produced by several soil microbes and is effective at mineral weathering. Our results analyze the K-organic salt bonding in detail to elucidate why the spectral shapes differ and indicate that the K K-edge XANES spectra are associated with the entire ligand despite similar first-shell bonding environments around the K center.